Hot-dip metal coating processes for steel strips have been standardized all over the world for over three decades and are based on a process introduced by the applicant in the early thirties and now known as the Sendzimir process. It consists in preliminary light oxidation of the surface followed by reduction in a hydrogen-rich atmosphere, cooling to a temperature slightly above the temperature of the molten metal and finally passing it through the molten metal bath. Zinc and aluminum are the two coating metals that are used in practice and both sides of the strip are coated. To meet a recent demand for strips galvanized on one side only, several proposals have been made by practically every steel company, and they all represent methods of adopting their existing Sendzimir lines to produce one-sided coatings. The methods fall into two classes: (1) applying a protective coating to the face that must not be coated, such as metallic oxides with or without a dilicate binder. U.S. Pat. No. 3,383,250, May 14, 1968, is illustrative of those disclosing such methods; and (2) instead of dipping the strip into the molten bath, letting only one face of the strip touch it flat and relying upon capillary attraction to "wet" that face with the molten zinc. Improvements of this method, involving electric or ultrasonic stirring of the molten metal to produce an upward-directed wave to "wet" that one face more easily, have also been patented and apparently are in operation. Several patents showing such arrangements have been issued to Armco Steel Corporation, and U.S. Pat. No. 4,152,471, May 1, 1979, is illustrative of such.
The defects or disadvantages of the known processes arise from the fact that the face that is to remain blank requires expensive treatment for either (1) the removal of the protective coating, or (2) the removal of the iron oxide coating that forms on that surface when it is exposed to atmosphere at the temperature of molten zinc.
Another disadvantage is that such processes involve a fixed heat treatment preceding the zinc coating and it is not the best treatment for deep drawing sheets, which is the purpose for which the one-side-coated sheets are required. Neither can temper-rolling be performed with precision after zinc coating.
In order to overcome the above deficiencies and also to create a superior product, applicant has developed a process that cannot be practiced on a modified Sendzimir line but which is, to the applicant's best knowledge, entirely novel and which, in addition, results in further important advantages.